Anyone who watches baseball should know its a pretty big leap to say the ballpark and defense will "just even out."

Over the course of a career? I'll say it again. The defense behind you will just even out, unless I guess if you play for the Yankees for your entire career.
I suppose John played on generally good teams, so he probably had somewhat better D, but not enough to make a huge difference.
Ballpark - again you play half your games on the road, these players play for several teams.
Regardless I don't see a lot of ballpark discounting for hitters.